Compact radio frequency ("RF") transmitters are widely employed in connection with remote signal communication systems, primarily for remotely controlling automatic garage door systems, electronic sound systems, televisions and VCRs. In the automotive industry, compact RF transmitters are commonly used in remote keyless entry systems to provide remote control access to a vehicle, as well as for enabling other vehicular functions including alarm system features and a trunk release, for example. Ideally, hand held transmitters are battery operated, energy efficient and intended to accommodate a compact enclosure.
In one known compact remote system design, an RF transmitter radiates an RF signal with a predetermined carrier frequency encoded according to an on/off switched pattern. This radiating signal is subsequently received by a remote receiver. Once received, the signal is processed, if necessary, and then provided as a control signal to control a function or feature of the system.
Currently, a number of compact remote RF transmitters employ a single oscillator design for providing a local oscillation signal. As illustrated in FIG. 1, a conventional transmitter circuit 5 is shown with a single oscillating circuit commonly referred to as the Colpitts oscillator. Transmitter circuit 5 generates a local oscillation signal which is transmitted from an antenna element L.sub.1. In light of its simplicity, circuit 5 has been the transmitter component of choice in automotive, remote controlled, keyless entry systems.
Referring to FIG. 1 in greater detail, the Colpitts oscillator of circuit 5 comprises a Colpitts configured transistor Q.sub.1, and an input resonant tank circuit. The tank circuit typically comprises a resonator, such as a surface acoustic wave ("SAW") device 2, and a pair of feedback capacitors, C.sub.1 and C.sub.2. Further, the oscillator also includes a number of biasing resistors to facilitate the proper operation of transistor Q.sub.1. Transmitter circuit 5 also comprises an inductor L.sub.1 which acts as an antenna element for radiating the RF output signal.
Structurally, transistor Q.sub.1, comprises a base 4, collector 6 and emitter 8. Base terminal 4 is coupled with surface acoustic wave resonator 2, and collector 6 is coupled with inductor L.sub.1, while emitter 8 is coupled to ground through a resistor R.sub.3. Additionally, feedback capacitor C.sub.2 is coupled between emitter 8 and ground, and as such, is in parallel with resistor R.sub.3. Feedback capacitor C.sub.1, is coupled between collector 6 and emitter 8. Moreover, a third capacitor C.sub.3 is coupled between inductor L.sub.1 and ground for providing a large capacitance to maintain a constant DC voltage.
Circuit 5, and more particularly L.sub.1 and C.sub.3, is coupled to a direct current ("DC") voltage source to receive a DC bias input V.sub.IN, typically 6V. Transmitter circuit 5 also receives a data input signal V.sub.DATA for encoding the RF carrier signal. As detailed hereinabove, circuit 5 generates a radiating output signal via inductor L.sub.1. In doing so, transistor Q.sub.1, acting as an amplifier, in combination with the resonating tank circuit, generates a resonating signal which is provided to inductor L.sub.1 as an oscillating current signal I. The conduction of current I through inductor L.sub.1 in turn causes the radiating output signal to be transmitted as an electromagnetic field.
The above described Colpitts oscillator is well suited for the RF signal transmission applications of a remote keyless entry system. However, such an oscillator design provides a limited amount of power output. Further, the alternative of a greater inductance value for radiating inductor L.sub.1 may not feasibly achieve a corresponding increase in power due to the inherent limitations of such components. Similar attempts to enhance output power through the optimization of component values has proved futile in view of the matching losses created thereby. Moreover, rail-to-rail voltage swings in transistor Q.sub.1 tend to confine the amount of current flow through the circuit which, in turn, diminishes the available power output realized by a given transmitter circuit.
As a result of the limited power available from compact remote transmitters using Colpitts oscillators, another problem has arisen with their application in compact remote transmitters. Typically, compact remote transmitters are hand grasped and directed generally toward a receiver of the system. By so doing, a parasitic impedance is created by the user's hand. This additional impedance reduces the amount of transmitted energy towards the receiver. This becomes an issue of particular significance in view of the limited power available from a traditional Colpitts oscillator.
Moreover, present compact remote transmitters employ a frequency shift key ("FSK") modulation scheme. Realizations of these designs have incorporated expensive components such as a PIN or varactor diode. One such FSK oscillator is depicted in U.S. Pat. No. 5,367,537. In these circuits, the PIN or varactor diode changes capacitance in response to a change in the control voltage applied. Unfortunately, this control voltage changes with the life of the battery supply voltage. As such, the center frequency of the FSK oscillator in turn drifts. This frequency drifting phenomenon is highly undesirable to the long term efficacy of a compact remote transmitter design.
In view of these problems, a need remains for a frequency shift key modulating oscillator circuit having a predictable center frequency which is not prone to drifting. A demand further exists for a frequency shift key modulating oscillator circuit which is more cost effective. Moreover, industry requires a frequency shift key modulating oscillator circuit drawing less energy from the power supply, and as such having an extended life span.